The present invention relates generally to voltage tunable varactors with associated DC blocking capacitors.
Varactors are voltage tunable capacitors in which the capacitance is dependent on a voltage applied thereto. This property can find applications in electrically tuning radio frequency (RF) circuits, such as filters, phase shifters, and so on. The most commonly used varactor is semiconductor diode varactor, which has the advantages of high tunability and low tuning voltage, but suffers low Q, low power handling capability, and limited capacitance range. A new type of varactor is a ferroelectric varactor capacitance is tuned by varying the dielectric constant of a ferroelectric material by changing the bias voltage. Ferroelectric varactors have high Q, high power handling capacity, and high capacitance range.
One ferroelectric varactor is disclosed in U.S. Pat. No. 5,640,042 entitled xe2x80x9cThin Film Ferroelectric Varactorxe2x80x9d by Thomas E. Koscica et al. That patent discloses a planar ferroelectric varactor, which includes a carrier substrate layer, a high temperature superconducting metallic layer deposited on the substrate, a lattice matching, a thin film ferroelectric layer deposited on the metallic layer, and a plurality of metallic conductors disposed on the ferroelectric layer and in contact with radio frequency (RF) transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194. Tunable varactors that utilizes a ferroelectric layer, and various devices that include such varactors are also disclosed in U.S. patent application Ser. No. 09/419,126, entitled xe2x80x9cVoltage Tunable Varactors And Tunable Devices Including Such Varactorsxe2x80x9d, filed Oct. 15, 1999, and assigned to the same assignee as the present invention.
When using such varactors in various devices, it is necessary to insert DC blocking capacitors in the RF transmission line to isolate the DC bias voltage from the other parts of RF system. These DC blocks on the transmission line may result in additional insertion loss to the RF system, and inconvenience in the design and construction of an RF system.
There is a need for varactors with reduced DC block insertion loss, but with high tunability, for use in the VHF, UHF, microwave and other tunable circuits, such as filters, phase shifters, voltage controlled oscillators and so on.
A voltage tunable dielectric varactor assembly constructed in accordance with this invention includes a tunable ferroelectric layer and first and second non-tunable dielectric layers. First and second electrodes positioned adjacent to the tunable ferroelectric layer form a tunable capacitor. The first and second electrodes are also positioned adjacent to the first and second non-tunable layers, respectively. A third electrode is positioned adjacent to the first non-tunable dielectric layer such that the third and first electrodes and the first non-tunable dielectric layer form a first blocking capacitor. A fourth electrode is positioned adjacent to the second non-tunable dielectric layer such that the fourth and second electrodes and the second non-tunable dielectric layer form a second blocking capacitor.
In one embodiment, the voltage tunable dielectric varactor includes a substrate having a generally planar surface and a tunable ferroelectric layer positioned on the generally planar surface of the substrate. First and second electrodes are positioned on a surface of the tunable ferroelectric layer opposite the generally planar surface of the substrate, with the first and second electrodes being separated to form a first gap. First and second non-tunable dielectric layers are also positioned on the generally planar surface of the substrate. A third electrode is positioned on the surface of the first non-tunable dielectric layer opposite the generally planar surface of the substrate such that the third and first electrodes form a second gap. A fourth electrode is positioned on the surface of the second non-tunable dielectric layer opposite the generally planar surface of the substrate such that the fourth and second electrodes form a third gap.
In another embodiment, the voltage tunable dielectric varactor includes a tunable ferroelectric layer and first and second non-tunable dielectric layers. The tunable layer is positioned between first and second electrodes to form a tunable capacitor. The first non-tunable layer is positioned between the first electrode and a third electrode to form a first blocking capacitor. The second non-tunable layer is positioned between the second electrode and a fourth electrode to form a second blocking capacitor.
Ferroelectric varactor assemblies of the present invention can be used to produce a phase shift in various microwave devices, and in other devices such as tunable filters.